Intramolecular cycloaddition/cycloreversion of (E)-3β,17β-diacetoxy-5,10-secoandrost-1(10)-en-5-one

Beyond olefins: new metathesis directions for synthesis

“…When Khripach and co-workers were attempting to protect the ketone functionality of E -70 as a dithioketal, they were surprised that upon exposure of macrocycle E -70 to the Lewis acid BF 3 ·Et 2 O (Scheme 14), they isolated cyclopentene 71 in 60%; a product they hypothesized resulted from an “unusual” intramolecular rearrangement. 29 After conformational analysis based on density functional theory calculations, they proposed a Lewis acid promoted [2+2] cycloaddition to form an oxetane intermediate, followed by a [2+2] cycloreversion reaction. The authors hypothesized that this mode of reactivity was a result of the steric arrangement of the ( E )-alkene and carbonyl group, as the same reactivity was not observed with the ( Z )-isomer of macrocycle 70 .…”

Tropylium-promoted carbonyl–olefin metathesis reactions

“…3 There might be several reasons for this, with one linked to the same versatile reactivity of the carbonyl functionality such that other chemical transformations often compete and overshadow the possible metathesis reaction. 2 , 3 Until recently, there were only a small number of stoichiometric Lewis acid-facilitated protocols 4 for the COM reaction ( Scheme 1a ) and relevant stoichiometric olefination reactions of carbonyl moieties. 5 …”

Synthesis and radical oxidation of steroidal 1-oxo-5α-alcohols

“…8 This limits the applicability of the reaction in a directed synthesis of corresponding seco steroids.…”